I’ve made a PKI issue, but I’m wondering if anyone else has run into this problem on High Sierra: I’m unable to install RStan because (I believe) shinystan depends on rsconnect which depends on PKI, and I can’t install PKI due to an OpenSSL headers issue.

Huh, running xcode-select --install fixed the problem. I guess I hadn’t redone that since upgrading to Sierra? (I definitely already have Xcode installed and have installed the command line tools before.) Funny that installing RStan is the first time I’ve noticed a problem from missing XCode command line tools, but at any rate, I’m all set now. Thanks!